1. Field of the Invention
The present invention relates generally to semiconductor memory cells and architectures and, more particularly, relates to a cell and architecture for an electrically erasable programmable read-only memory (E.sup.2 PROM)
2. Description of the Relevant Art
The potential for E.sup.2 PROM memory arrays to replace standard nonvolatile memory devices, such as disks, tapes, etc., has stimulated research into new architectures for E.sup.2 PROMs. As is well known, the cost associated with electronic nonvolatile memories facilitated on integrated circuit (IC) semiconductor chips are much less than associated with memories requiring mechanical components. However, at present E.sup.2 PROM memory architectures have not achieved great commercial success.
Typical E.sup.2 PROM memory cells are complex structures requiring the precise positioning of diffusion regions, oxide isolation regions, thin oxide layers, and polycrystalline silicon elements. Additionally, the various voltage levels required for read, program, and erase operations require connecting lines to provide externally-generated voltage levels. Accordingly, the density, or number of memory cells/unit area, of existing E.sup.2 PROM architectures is low.
The program and erase operations in an E.sup.2 PROM generally require high voltage levels to charge and discharge the floating gates in the E.sup.2 PROM cells. These high voltage levels are difficult to generate on the IC chip, make isolation of the diffusion regions on the surface of the chip difficult, and cause bipolar latch-up problems in CMOS peripheral circuits on the chip.
Further, E.sup.2 PROM capacitance of the word lines and bit lines in these arrays is generally large. These bit lines and word lines must be charged to selected bias voltage levels during a read operation. The high capacitance increases the time required to bias the word lines and bit lines and decreases the speed of the memory array Read Access Time.
Accordingly, an E.sup.2 PROM architecture that is fast, facilitates high density, and does not require high voltage levels during the program and erase operations is greatly needed in the semiconductor industry.